【作者】 王文静；

【导师】 孟维晓；

【作者基本信息】 哈尔滨工业大学 ， 信息与通信工程， 2007， 硕士

【摘要】 目前,GPS卫星信号采用两种扩频伪码序列,分别是用于标准定位服务的C/A码和用于精确定位服务的P码。相比于C/A码,P码不仅定位精度高,且具有很强的抗干扰和反欺骗能力。传统的P码捕获技术需利用C/A码得到下一子帧的起始时间,以完成P码相位的同步。当C/A码受到攻击时,只能利用P码直接捕获技术完成导航定位服务。然而,由于P码速率高且周期长,使得直接捕获的难度较大,已成为GPS导航定位领域中的一项技术难点。因此,研究P码的直接捕获技术对完善我国自己的卫星导航定位系统有着重要的意义。本文首先给出了GPS信号基本结构及其调制方式,然后对伪码捕获技术进行了介绍,其中包括基于FFT的伪码相位捕获技术。与此同时,阐述了比特翻转以及多普勒频移对捕获过程产生的影响,并推导了可用于P码捕获的最大数据长度。接着详细描述了各种P码捕获方案原理及其算法结构,其中包括FFT块处理结合频域并行搜索的捕获模型结构。在此基础上,还研究了三种能够有效减少FFT运算量的块处理方式,并根据这三种方案的实现过程进行仿真实验,给出了各方案的捕获仿真结果。同时,还以列表的方式比较了不同参数下各方案所需的平均捕获时间。其次,针对已有算法普遍存在的峰值模糊问题,本文提出了一种改进算法。结合经典算法的捕获模型,对改进算法的总体结构进行了设计。接着详尽地阐述了改进算法中Q路频谱提取法和保留叠加法的实现原理。给出了各多普勒补偿支路的相关峰值随载波频率偏移的变化曲线,验证了Q路频谱提取法对峰值模糊问题的改善效果。同时,也给出了相关峰值随码相位偏移的变化曲线,验证了保留叠加法对相关峰值的提升效果。并通过仿真,对改进算法和经典算法的检测概率、频偏误判概率进行了对比分析。最后,本文对改进算法的捕获过程以及各项捕获指标进行了仿真,分别给出了码相位和多普勒频偏的捕获结果。然后,分析了虚警概率与判决门限的关系、检测概率与SNR的关系、捕获时间与选取码段长度的关系,给出了不同SNR下的自适应门限系数,通过比较改进算法和经典算法信噪比增益曲线,验证了改进算法的优越性。更多还原

【Abstract】 As it stands today, GPS adopts two types of pseudorandom noise code spreading sequences, coarse/acquisition code for Standard Positioning Service and precision code for Precise Positioning Serivice, respectively. Compared with coarse/acquisition code, precision code can not only provide high precise positioning, but also has high tolerance to jamming and spoofing. Conventional approaches of precision code acquisition use coarse/acquisition code to obtain the starting of next subframe in order to realize phase synchronization of precision code. When coarse/acquisition code is degraded during certain military operations, direct acquisition of P-code is the only option available to maintain navigation and positioning services. However, it is difficult to achieve direct acquisition of precision code because of its high chip rate and long period, which already becomes a technical nodus in the field of GPS. Consequently, it is very meaningful for the improvement on the satellite navigation and positioning system of our country to make research on precision code direct acquisition technology.Firstly, this thesis introduces the structure of the GPS signal and its modulation scheme. Then, various approaches of precision code direct acquisition is summarized, which includes acquisition methods based on FFT. Meanwhile, the influence of bit reversal and Doppler frequency shift on the acquisition process is expatiated and the maximum data length for precision code acquisition is evaluated. Then the principle and algorithm structure of various precision code acquisition schemes are described in detail, which includs the acquisition method of joint FFT block processing and frequency domain parallel search. Three block processing methods which can effectively reduce the amount of FFT operations are researched, and corresponding simulations are performed. The average acquisition time of these schemes are also listed and compared.Secondly, an improved algorithm is proposed aiming at the ubiquitous problems of ambiguous peak widely encountered in existing algorithms of precision code direct acquisition. Based on the acquisition model of conventional algorithms, the overall structure of the improved algorithm is designed. The improved algorithm is described in detail, which includes the approach of extracting the Q branch’s spectrum and the hold-overlapping method. The curve of correlation peak value of each Doppler compensation branch versus carrier frequency shift is shown, which proves the improved performance of the extracting the Q branch’s spectrum approach corresponding to the ambiguous peak problem. Meanwhile, the curve of correlation peak value versus code phase shift is given, which proves the effect of peak value enhancement of the hold-overlapping method. Simulations have been done in order to compare and analyze the detection probability and the error estimation probability of carrier frequency offset between the improved and conventiaonal algorithm. Comparisons between the improved and conventional algorithms in the aspect of detection probability and the error estimation probability of carrier frequency shift are made through simulations.Finally, the overall acquisition process and various acquisition parameters of the improved algorithm are simulated, and the acquisition results of code phase and Doppler frequency offset are shown respectively. Then the relationships between false alarm probability and estimation threshold, detection probability vs SNR, acquisition time vs code length are analyzed, and the adaptive threshold coefficients under different SNRs are given. The superiority of the improved algorithm is is proved by comparing the SNR gain curves between the improved and conventional algorithms.更多还原